Tubular spring for actuator, and method for assembling the tubular spring

ABSTRACT

A tubular spring is provided for preloading a piezoelectric or magnetostrictive actuator for the actuation of fuel injection valves for fuel injection systems of internal combustion engines. The tubular spring has at least two assembly engagements for uniform preloading of the tubular spring.

FIELD OF THE INVENTION

The present invention relates to a tubular spring for an actuator and amethod for assembling the tubular spring.

BACKGROUND INFORMATION

Piezoelectric or magnetostrictive actuators for the actuation of fuelinjection valves for fuel injection systems of internal combustionengines are usually preloaded by way of a spring in order to eliminatetensile and shear forces. The residual load of the fuel injectionvalve's return spring, present even when the fuel injection valve isclosed, is usually used to generate the preload.

An actuator, e.g., for the actuation of fuel injection valves for fuelinjection systems of internal combustion engines, that has multiplelayers made of a piezoelectric or magnetostrictive material arranged instacked fashion, is described in published German patent document DE 19951 012. The actuator is preloaded by way of a tie rod disposed in acentral recess, by an opposite-direction thread. Immobilization andenergy transfer are accomplished via a cover plate and a base plate.

This configuration is disadvantageous in particular because of the needto configure the stacked actuator in hollow fashion so that the tie rodcan be accommodated in the recess. This makes the actuator even moresusceptible to damage during preassembly and assembly.

Described in published international patent document WO 99/08330, is apiezoelectric actuator that is slid into a spring sleeve and joined, inpreloaded and in frictionally or positively engaged fashion, to two endsof the spring sleeve. The result is to produce a basic unit in which thepreload force of the piezoelectric actuator is permanently defined.

The particular disadvantage of this actuator is that the preload of theactuator is defined solely by the elasticity of the spring sleeve.Progressive characteristics in order to compensate for short activationtimes cannot thereby be achieved.

SUMMARY

The tubular spring according to the present invention for an actuator,and the method according to the present invention for assembly, have theadvantage that assembly engagements in the tubular spring make possibleuniform preloading of the tubular spring and of the actuator, andstress-free welding of the tubular spring to the actuator head andactuator foot.

Advantageously, the assembly engagements are disposed in at least one ofthe otherwise continuous end regions of the tubular spring. The result,on the one hand, is that the spring characteristics of the tubularspring are not negatively influenced. A positive influence, with alonger possible actuator stroke, is possible as a result of introductionof the assembly engagements in only one end region.

It is additionally advantageous that the assembly engagements aredisposed on the tubular spring symmetrically and in paired fashion, orequidistantly at identical angular spacings, so that uniform loading ispossible.

It is furthermore advantageous that a preassembly of the actuator modulein the load-free state is first performed before the actuator module isinserted into an assembly apparatus for preloading and final assembly.As a result, the actuator is already protected from damage due to shearforces, and can be stored and transported until further processed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a fuel injection valvesuitable for being fitted with a tubular spring according to the presentinvention.

FIG. 2 is a plan view of a sheet-metal blank for manufacture of thetubular spring, according to a first example embodiment.

FIG. 3 is a schematic cross-sectional view illustrating the assembly ofa tubular spring configured according to the present invention as shownin FIG. 2.

FIG. 4 is a perspective view of the completely assembled tubular springconfigured according to the present invention as shown in FIGS. 2 and 3.

FIG. 5 is a plan view of a sheet-metal blank for manufacture of thetubular spring, according to a second example embodiment.

FIG. 6 is a side view of the rolled-up sheet-metal blank shown in FIG.5, of the tubular spring configured according to the present invention.

FIGS. 7A and 7B depict, in a sectional view and a side view,respectively, the completely assembled tubular spring configuredaccording to the present invention as shown in FIGS. 5 and 6.

DETAILED DESCRIPTION

FIG. 1 shows, in a highly schematic depiction, a fuel injection valvesuitable for being fitted with a tubular spring configured according tothe present invention. Fuel injection valve 1 depicted in FIG. 1 issuitable, in particular, as a fuel injection valve 1 for directinjection of fuel into a combustion chamber (not depicted further) of aninternal combustion engine.

Fuel injection valve 1 encompasses a nozzle body 2 in which a valveneedle 3 is guided. The latter has at one outflow end a valve closureelement 4 that forms a sealing fit with a valve seat surface 5. Fuelinjection valve 1 is embodied as an outward-opening fuel injection valve1. A return spring 6, disposed in nozzle body 2, impinges upon valveneedle 3 in such a way that fuel injection valve 1 is held closed in theidle phase, and valve needle 3 is returned back to its idle positionafter the opening phase.

Nozzle body 2 opens into a housing 7 in which a hydraulic coupler 8 andan actuator module 9 are disposed.

Actuator module 9 encompasses a piezoelectric stacked actuator 10 that,for protection against shear forces, is preloaded with a tubular spring11 configured according to the present invention. In a present exampleembodiment, actuator 10 is additionally protected by an injection-moldedsheath 12. Actuator 10 has an actuator foot 13 and an actuator head 14which are, for example, welded to tubular spring 11. Tubular spring 11and the components of actuator 10 are described in more detail belowwith reference to FIGS. 2 through 4. Hydraulic coupler 8, which isdisposed on the inflow side of actuator module 9, encompasses a workingpiston 15 that is preloaded by a coupler spring 16 with respect tohousing 7. Hydraulic coupler 8 converts the short stroke ofpiezoelectric actuator 10 into a longer stroke of valve needle 3.Coupler 8 and actuator module 9 are encapsulated in an actuator housing17. Flowing around the latter is the fuel, delivered centrally via aninflow fitting 18, that flows through housing 7 and through nozzle body2 to the sealing seat. Fuel injection valve 1 is actuatable via anelectrical plug contact 19.

In order to ensure, during the assembly and operation of actuator 10,that shear forces which may result in damage to actuator 10 areprevented, actuator 10 is encapsulated in tubular spring 11. The latterpreloads actuator 10 so that it is stabilized by the exclusively axialforces acting on it.

Tubular spring 11 is manufactured in the manner depicted in FIGS. 2 and5 for two different example embodiments, respectively. Firstly, a basicshape is produced by punching out of a sheet-metal blank 20, and isrolled up as depicted in FIG. 6 for the second example embodiment. In anabutting region in which edges 21 of tubular spring 11 are located nextto one another, those edges are joined to one another by longitudinalwelding so that a closed tube is created.

During manufacture of the basic shape, cutouts 22 are introduced intosheet-metal blank 20. Cutouts 22 are regularly distributed over aportion of the surface of tubular spring 11, cutouts 22 being embodiedapproximately in the shape of an “8”. Cutouts 22 are produced bypunching or similar methods. Webs 23 are left behind between cutouts 22as a result of the machining process, and are responsible for thesupporting and resilient action of tubular spring 11. As a result of the“8”-shape of cutouts 22, tubular spring 11 is flexible in terms of itsaxial extension: it can easily be extended or compressed. The springconstant of tubular spring 11 is influenced by the number, shape, size,and placement of cutouts 22. According to the first example embodimentdepicted in FIG. 2, cutouts 22 are embodied in a central region 32 ofthe tubular spring, whereas end regions 33 remain free of cutouts 22and, aside from assembly engagements 24 described in more detail below,have a continuous surface. In the second example embodiment depicted inFIGS. 5 and 6, cutouts 22 are embodied both in central region 32 and inone of end regions 33.

Assembly engagements 24 that are required for the assembly of actuator10 in tubular spring 11 are provided according to the present invention.In the first example embodiment depicted in FIG. 2, assembly engagements24 are provided in both end regions 33 of tubular spring 11, whereasaccording to the second example embodiment depicted in FIGS. 5 and 6,assembly engagements 24 are embodied in only one of end regions 33. Theregion in which cutouts 22 are embodied is larger as a result, so thattubular spring 11 becomes more elastic.

As is apparent from the first example embodiment depicted in FIG. 2, foruniform force introduction, assembly engagements 24 are provided here atleast in paired fashion opposite one another. In the second exampleembodiment depicted in FIGS. 5 and 6, on the other hand, at least threeassembly engagements 24 are present, distributed equidistantly over thecircumference of tubular spring 11 at identical angular spacings.Between assembly engagements 24, further cutouts 34 can be providedwhich, like the additional number of cutouts 22, make tubular spring 11softer and thus enable a longer stroke for actuator 10. The function ofassembly engagements 24 is explained in more detail in the descriptionrelating to FIGS. 3, 7A and 7B.

FIG. 3 depicts the assembly of actuator 10 in a tubular spring 11 shownin FIG. 2.

Actuator 10 is advantageously assembled in tubular spring 11 in such away that a preload is generated in actuator module 9 by acontrolled-force displacement. Despite production and stiffnesstolerances, the preload force can be adjusted very accurately withoutthe use of further components. The welding of actuator head 14 andactuator foot 13 to tubular spring 11 can be accomplished in a regionunaffected by the preload force, so that the weld can be created instress-free fashion and exhibits considerably better stability. This ismade possible by the stiffness of tubular spring 11 in the region ofassembly engagements 24.

Assembly is performed in the following sequence: First, tubular spring11 is fitted onto actuator foot 13 and welded to it with a first weldseam 25. Then actuator 10 is inserted into tubular spring 11. Whileactuator 10 is held in contact against actuator foot 13, centering pins26 are inserted between electrical lines 19 and tubular spring 11 inorder to center actuator 10 in tubular spring 11. Actuator head 14 isthen inserted, and slid into tubular spring 11 until it rests againstactuator 10. Actuator module 9 preassembled in this fashion istransportable without difficulty, since the sensitive actuator 10 islocated in tubular spring 11 and protected from shear forces. Actuatormodule 9 is inserted into an assembly device 27 that is depictedschematically in FIG. 3, and clamping jaws 28 engage into outflow-endassembly engagements 24 of tubular spring 11. A load cell 29 restsagainst actuator head 14. Load cell 29 is first calibrated, and thenclamping jaws 28 along with tubular spring 11 are displaced toward loadcell 29, as indicated by arrows 30. The force F with which tubularspring 11 is pressed against actuator head 14 can be read off at anytime directly from load cell 29. Tubular spring 11 can then be displaceduntil the desired preload on actuator 10 is achieved. The magnitude ofthe force is based on requirements regarding the stroke of actuator 10,and the desired activation characteristics. A progressive springconstant can also be attained with a suitable configuration of tubularspring 11.

Lastly, tubular spring 11 is welded to actuator head 14 via a secondweld seam,31. The force path in tubular spring 11 is such that allcomponents that are later to be under load are also stressed duringpreloading. Only end region 33 of tubular spring 11 in which assemblyengagements 24 are embodied is not located in the force path, so thatsecond weld seam 31 can be created in a stress-free state.

FIG. 4 shows the completely assembled actuator module 9. Tubular spring11 is welded by way of weld seams 25 and 31 to actuator foot 13 andactuator head 14, respectively. Assembly engagements 24 are requiredonly in end region 33 of tubular spring 11 located closer to actuatorhead 14, but may nevertheless be provided at both ends of tubular spring11 as also depicted in FIGS. 2 and 3, both for reasons of symmetry inorder to avoid any distortion of the spring characteristic and for costreasons.

FIGS. 7A and 7B depict a completely assembled actuator module 9 having atubular spring 11 as shown in FIG. 6, in a sectional view and a sideview, respectively.

The assembly process for a tubular spring 11 according to the secondexample embodiment, depicted in FIGS. 5 through 7, encompasses thefollowing assembly steps. First, actuator 10 is fitted onto a fuel inlet18 joined to an actuator foot 13. Tubular spring 11 is then likewisefitted onto actuator foot 13, and joined to it in stress-free fashionvia a first weld seam 25. Tubular spring 11 is then preloaded by tensionon assembly engagements 24. Tubular spring 11 is first overextendedbeyond the necessary tensile force, and the tensile force is thenreduced to the necessary tensile force of approximately 600 N.

Lastly, tubular spring 11 is welded to actuator head 14 via a secondweld seam 31. The force path in tubular spring 11 is such that allcomponents that are later to be under load are also stressed duringpreloading. Only end region 33 of tubular spring 11 in which assemblyengagements 24 are embodied is not located in the force path, so thatsecond weld seam 31 can be created in a stress-free state.

The present invention is not limited to the example embodimentsdepicted, and is applicable in particular to a plurality of fuelinjection valve designs. All the features of the example embodiment mayalso be combined with one another.

1-19. (canceled)
 20. A tubular spring for one of a piezoelectricactuator and a magnetostrictive actuator for actuation of a fuelinjection valve of a fuel injection system, comprising: a tubular body,wherein the tubular body is in operative engagement with the one of thepiezoelectric actuator and the magnetostrictive actuator, and whereinthe tubular body has at least one assembly engagement in the form of acut-out in an axial end region of the tubular body, and wherein the atleast one assembly engagement enables uniform preloading of the tubularspring and the one of the piezoelectric actuator and themagnetostrictive actuator.
 21. The tubular spring as recited in claim20, wherein the tubular spring has, at least in a central region,cut-outs and webs located between the cut-outs.
 22. The tubular springas recited in claim 21, wherein at least one assembly engagement isprovided at each axial end region of the tubular spring.
 23. The tubularspring as recited in claim 21, wherein at least two assembly engagementsare provided at each of the axial end regions.
 24. The tubular spring asrecited in claim 23, wherein at each axial end region of the tubularspring, the two assembly engagements are disposed radially opposite oneanother.
 25. The tubular spring as recited in claim 21, wherein the atleast one assembly engagement is only provided at a first axial endregion of the tubular spring.
 26. The tubular spring as recited in claim25, wherein a second axial end region of the tubular spring is providedwith cut-outs and webs located between the cut-outs.
 27. The tubularspring as recited in claim 25, wherein at least three assemblyengagements are provided at the first axial end region of the tubularspring.
 28. The tubular spring as recited in claim 27, wherein the atleast three assembly engagements are separated by a regular annularspacing.
 29. The tubular spring as recited in claim 28, whereinadditional cut-outs are provided between the assembly engagements. 30.The tubular spring as recited in claim 21, wherein the tubular spring isconnected to an actuator head and to an actuator foot of the one of thepiezoelectric actuator and the magnetostrictive actuator.
 31. Thetubular spring as recited in claim 30, wherein connections to theactuator head and to the actuator foot are achieved by weld seams.
 32. Amethod for producing an actuator module for a fuel injection valve of afuel injection system, the method comprising: a) providing a tubularspring having at least two assembly engagements in the form of cut-outs;b) assembling an intermediate module including the tubular spring, anactuator, an actuator foot and an actuator head; c) inserting theintermediate module into an assembly apparatus having at least twoengagement arms for engaging the at least two assembly engagements; d)preloading the tubular spring by applying tension on the at least twoassembly engagements; and e) affixing the tubular spring to the actuatorhead.
 33. The method as recited in claim 32, wherein step b) includes:fitting the tubular spring onto the actuator foot; welding the tubularspring to the actuator foot with a first weld seam; introducing theactuator into the tubular spring; holding the actuator in contact withthe actuator foot; inserting at least two centering pins between thetubular spring and electrical lines disposed in the actuator foot; andinserting the actuator head into the tubular spring until the actuatorhead rests against the actuator.
 34. The method as recited in claim 33,wherein step c) includes: inserting the intermediate module into theassembly device; placing the at least two engagement arms in the form ofclamping jaws into the assembly engagements of the tubular spring;providing a load cell in contact with the actuator head; and calibratingthe load cell.
 35. The method as recited in one of claim 34, whereinstep d) includes: displacing the clamping jaws together with the tubularspring toward the load cell; determining the force from the load cell;and welding the tubular spring to the actuator head with a second weldseam.
 36. A method for producing an actuator module for a fuel injectionvalve of a fuel injection system, the method comprising: providing atubular spring having at least two assembly engagements in the form ofcut-outs; fitting an actuator onto a fuel inlet joined to an actuatorfoot; fitting and welding the tubular spring onto the actuator foot witha first weld seam; preloading the tubular spring by applying tension onthe assembly engagements; and fitting and welding the preloaded tubularspring onto an actuator head with a second weld seam.
 37. The method asrecited in claim 36, wherein in preloading the tubular spring, thetubular spring is first overextended by applying a tensile force beyonda necessary tensile force, and the applied tensile force is subsequentlyreduced to the necessary tensile force.
 38. The method as recited inclaim 37, wherein the necessary tensile force is approximately 600 N.